This invention relates to the measurement of ultrafine particle size distributions, and in particular to measurement equipment useful in measuring the effectiveness of microelectronic manufacturing facility clean room equipment in maintaining the facility to the required or desired freedom from dust particles.
A modern microelectronics clean room requires continuous monitoring to insure that the required air quality is maintained. Microelectronic circuit elements have become so small (into the submicron range) that ultrafine dust particles (those smaller than 0.2 um) can cause circuit failure so it has become necessary to be able to measure ultrafine aerosol particles, at concentrations of less than 100 per cubic foot in order to adequately monitor clean rooms for manufacturing small microelectronic components. These low concentrations of ultrafine particles can not be accurately measured nor their distribution determined by present equipment.
While it is known, for example, to use diffusion batteries to separate various ultrafine particle sizes in an air sample in order to be able to measure the particles, conventional methods of doing so sequentially measure particle populations in size increments. Such an approach is discussed in "Measurement of Nanometer Aerosols" by David Sinclair, Aerosol Science and Technology, volume 5, Number 2, 1986, pp. 187-203 which discusses the use of a rotary sequencing valve in combination with a single diffusion battery and a CNC. We have determined that at low concentrations the time required to collect a statistically significant number of particles is so long that the size distribution of the aerosol may change before the measurement in such an approach is complete.
Ultrafine particles are too small to be detected by today's optical particle counters or by laser particle detectors (which cannot detect particles smaller than 0.05-0.1 micrometers). Ultrafine particles can be counted, however, without size classification, by commercial condensation nucleus detectors (CNCs). CNC's operate on the scientific principle that a supersaturated vapor will condense on small particles, forming larger droplets that make detection easier. This phenomena occurs naturally on foggy days or upon the formation of clouds in the atmosphere. One CNC is disclosed and described in U.S. Pat. No. 4,790,650 issued Dec. 13, 1988 to Patricia B. Keady, and assigned to TSI Incorporated, which patent is incorporated herein by reference. Also see "Continuous Flow, Single-Particle-Counting Condensation Nucleus Counter," Journal of Aerosol Science, Vol. 11, pp. 343-357 by Jugal K. Agarwal and Gilmore J. Sem.
Prior art proposals have included the use of electrostatic classifiers in combination with CNC detectors. An electrostatic classifier separates particles according to electrical mobility. However, the use of a particle size classifier has not proven to be practical and effective for very low levels of ultrafine particles, again because of long sequential counting times.
None of the various past approaches have provided an adequate measurement of the ultrafine particle size distributions in modern microelectronic circuit fabrication clean rooms. The configuration disclosed in this invention allows measurement of ultrafine particles over extended periods and during short bursts or short term (a few seconds) increases in concentration.